1. Field of the Invention
The present invention relates to a diplexer which can be used for a transceiver or the like.
2. Description of the Related Art
FIG. 5 shows a conventional diplexer. The diplexer has a low pass filter circuit comprising coils L1 and L2 and capacitors C1 to C4 and a high pass filter circuit comprising a coil L3 and capacitors C5 to C7. One end of the low pass filter circuit and one end of the high pass filter circuit are connected with each other to serve as a common terminal. The other end of the low pass filter circuit serves as a low pass filter terminal, and the other end of the high pass filter circuit serves as a high pass filter terminal. In the low pass filter circuit, a parallel resonance circuit having a resonance frequency higher than a second frequency is provided near the common terminal and comprises the coil L1 and capacitor C1 which are connected in parallel with each other.
Further, a parallel resonance circuit having a resonance frequency twice as large as a first frequency (the range of from 880 to 960 MHz) or the second frequency (the range of from 1710 to 2170 MHz) is provided near the low pass filter terminal and comprises the coil L2 and the capacitor C2 which are connected in parallel with each other. Then, the parallel resonance circuits are connected with each other. The capacitor C3 is provided at the connection of the parallel resonance circuit comprising the coil L1 and the capacitor C1 and the parallel resonance circuit comprising the coil L2 and the capacitor C2, and the capacitor C4 is provided near the low pass filter terminal. Each of the capacitors C3 and C4 serves a ground capacitor.
Further, in the high pass filter circuit, the capacitors C5 and C6 are connected in series between the common terminal and the high pass filter terminal. The connection of the capacitors C5 and C6 is grounded via a series resonance circuit comprising the coil L3 and the capacitor C7. Accordingly, the high pass filter circuit with an attenuation pole is constructed (for example, see Japanese Unexamined Patent Application Publication No. 2003-069362 (FIG. 1)).
In the above-mentioned configuration, when the low pass filter circuit transmits signals in a plurality of different frequency bands and similarly the high pass filter circuit transmits signals in a plurality of different frequency bands, for example, the low pass filter circuit does not sufficiently reflect the signals in the respective frequency bands which the high pass filter transmits, and the high pass filter circuit does not sufficiently reflect the signals in the respective frequency bands which the low pass filter transmits. Thus, it becomes difficult to transmit the signals in the plurality of different frequency bands without loss.